The present invention relates to an automatic document feeder (ADF) for a facsimile apparatus, printer or similar image forming apparatus and capable of feeding a stack of documents one by one while separating them from each other.
An ADF extensively used today has a document table to be loaded with a stack of documents. A feed roller and a roller contacting it cooperate to feed one document at a time while separating it from the others. One of major problems with the ADF is that the document separating ability greatly depends on how the operator sets documents on the document table. For example, when documents are inserted into the ADF as far as their leading edges reach the gap between the two cooperative rollers, it is likely that two or more of them are fed at the same time.
Some different approaches have been proposed to eliminate the above problem. For example, Japanese Patent Laid-Open Publication Nos. 63-123740 and 3-166128 teach a stop which aligns the leading edges of documents at a predetermined position by causing them to abut against it. This successfully prevents the leading edges of document from reaching the gap between the feed roller and the roller contacting it. In addition, such an approach obviates the simultaneous feed of two or more documents by aligning the leading edges of documents.
However, in the above Laid-Open Publication No. 3-166128, a shutter torque limiter continuously slides during the course of document feed in order to generate a torque for raising the stop. This forces a motor to bear an extra load corresponding to the torque being generated by the shutter torque limiter. Hence, an expensive high-output motor is needed. Further, the torque limiter brings about a durability problem since it constantly slides.